Photocell controlled safety system for fuel burners



m- 1953 E c. THOMSON 2,624,398

PHOTOCELL CONTROLLED SAFETY SYSTEM FOR FUEL BURNERS Filed Aug. 22, 1947 lnvenfnr E. arm/a THOMSON l Ql l FM/p E). 1 Ag ig Patented Jan. 6, 1953 UNITED STATES PATENT OFFICE PHOTOCELL CONTROLLED SAFETY SYSTEM FOR, FUEL BURNERS E. Craig Thomson, Boston, Mass, assignor to Gom-bustion Gontrol Corporation, Cambridge, Mass, a corporation of Massachusetts AppIibat'ion-AugustZZ, 19.47,. Serial No. 770,088

This invention relates generally to safety devicesusedfor flame failure detectionin domestic and industrial heating plants and more particulariy'to an electronic circuit suitable for devices in which a photocell serves as the flame detect- 'ing' element, the present invention being an improvement on the apparatus disclosed in the inventors Patent No. 2,416,781, issued March 4, 1947', and adaptable for use in any similar cir- 'cuits.

Photoelectric flame failure controls of the type herein under consideration are designed to prevent furnace explosions by shutting down the fuel supply'system of a burner when the heme is "extinguished. The photocell is so arranged .as tofbe illuminated by the flame when the burner isin normal operation. Under this condition the current derived. from the photocell, through its associated amplifier circuit, energizes a relay or similar device which in turn maintains the fuel supply system in operation. When fi'amejfa'ilure occurs, the drop in current through the photocell, due to absence of'light, results in de-energization of the relay and shutdown of'the fuel supplysysteni. Such controls are customarily enciosed' in ahousing which not onlyprotect's the electrical components from dirt, dampness, and tampering; .butpalso' serves to exclude light from sources other than the furnace flame. It is apparent that,.if the photocell were sufiicientlfy i1- luminatedirom" an extraneous source, the relay would remain" energized in the absence of flame and so permit unsafe operation of" the burner; For this reason; it is desirable to" provide some means for'automatically de-en'ergizing'the relay when the cover of the" housing is removed for servicing or inspection purposes; It'is', of course, possible to accomplish this result in. a conventional manner by'providing a cover switch which interrupts the power supply to the control when the cover is removed; Such an arrangement often may be" objectionable in this type of control, however, because the heaters of any electronic=- tubes included in the controlcircuit are allowed" to "cool, occasioning some delay in put ting the burner into operation when the cover is'repl'ace. Furthermore, a switch designed to interrupt a high voltage, high current, power line is necessarily more cumbersome and expensive, and" deteriorates more rapidly; than" a switchrwhich is required to interrupt only a' low current circuit;

The objects of'this invention. are, accordingly, to provide; in a photoele'ctric" flame" failure control; a-cover' switch connection which servesto shut down the burner when the cover of the control is removed, but nevertheless permits-the heaters of any amplifier tubes in the controlcircuit to remain energized, which renders the burner immediately operative when thecover is replaced, and which requires interruption of onlya small current by the cover switch.

In order that all of the features of this vention andthemode of operation thereof may be readily understood, detailed description follows hereinafter, with particular "reference being made to thedrawings wherein:

Fig. 1'- is a schematic circuit diagram of a preferred' embodiment of this invention shown in operative relationship with a portion of .acombustion chamber which is diagrammatically represented; and

Fig. 2' is a view showing a housing-fort-he electronic apparatus of thisinvention with-thecover' safety switch shown in detail.

Referring now particularly to Fig. 1 a transformeriii comprises a primary winding H supplied at terminals 1 and 'i'5-with power from an alternating current source (not shown)", a 10W voltage secondary winding It, for energizing the heater i1 of'an electron discharge tube It", and high voltage secondary winding l'2. While other types of tubes maybe used in the circuit, tube It is here shown as a pentode, having three grids f9, 26, and 2!, conventionally designated the control,v screen and. suppressor grids; respectively. The terminal 23 of secondary l2 isconnected, through a resistance 25 and arelay 2 6', both bridged by a smoothing condenser- 2=1:,.--to theanode 22 0f tube 16s The terminal Misconnectedthrough a capacitanceE-B-to the cathodetil of a phototube 29. On secondary J2, thereais -a tap 32 which is connected to the-cathode. 1840f tube t6, and between tap 32 v and terminal ;23athere is another tap 33' which is connected to ground and'to the anode 31 of phototube 2J9. 'The-icon trol grid, 19 of tube Leis connected-througha resistance-=34 to 'the phototube cathode 3lrand the suppressor gridlZl istconnecteditothe' cathode 'ld; A cover switch 36 is-connected between tap-33 and the screen grid 20 of; the discharge tube 46; A resistance 35 is connected between thesc'reeh grid 20 andthe cathode [8:01 the discharg'e' 'tube Hi. Relay 2& through contact 31 controls-the circuits to zburner'imotcnta and fuel valve-Minter initial starting in response to the momentary manual closure of pushbutton switch'4'4-.- It is, of: course, understoodthatthe burner-motor and valve may besubject additionally to control by any well-known means such as a thermostat or limit switch (not shown).

It is contemplated that the circuitry of this invention will be enclosed within a housing substantially identical to the two-piece housing shown in Fig. 2. Housing portions 45 and Z6 isolate phototube 29 so that the only light impinging thereupon passes through coupling pipe 41. In a practical installation, a pipe extension coupled to pipe 41 projects through hole 4| located in wall 42 of combustion chamber 40, so that burner flame 39 is monitored by photoelectric cell 29.

Safety switch 36 is mechanically coupled to a bracket afiixed to housing portion 46 so that removal of housing portion 45 disassociates metallic projection 48 from the plunger element of switch 36. Accordingly, the removal of housing portion 45 from 45 is necessary for the closure of switch 36.

The operation of the circuitry of Fig. 1 is as follows: 7

Initially, pushbutton switch 64 is manually closed so that the energizing potential applied to terminal i is applied to fuel valve 43 and burner motor 33. The fuel appearing at the burner nozzle is manually ignited by means of a torch or the like. Thereafter, in response to the detection of fire flame 39 by phototube 29, relay 26 is energized closing contact 3? and thereby shunting pushbutton switch at. With this occurrence, pushbutton switch 44 can be released without affecting the continued operation of the circuit. The details pertaining to the energization of relay 26 in response to flame detection by phototube 29 are hereinafter set forth.

On the half cycle when terminal 24 is positive, grid I9 is positive with respect to cathode l8 and grid current flows, charging capacitance 28. No current will flow through the phototube 29 on this half cycle since the phototube anode 3! is negative with respect to phototube cathode 30. On the next half cycle, when terminal 23 is positive, tap 32 is positive with respect to terminal 2 3, and tap 33 is positive with respect to both tap 32 and terminal 24. If phototube 29 is illuminated, it passes current which discharges capacitance 28, thereby reducing the negative bias on control grid I9 to a value insufiicient to cut off current through the tube. Since screen grid 23 is positive with respect to cathode l8, current flows in the anode-cathode circuit, energizing relay 26, contact 31 is closed, maintaining motor 38 and valve 43 energized. If the flame is extinguished, accidentally or by some control means, the phototube 29 is no longer illuminated. During the half cycle when terminal 23 is positive, therefore, only negligible current flows through the phototube 29. Consequently, the charge built up on capacitance 28 during the previous half cycle is not removed, and this charge added to the potential supplied by the section of secondary I2 between terminal 24 and tap 3.2, is suflicient to maintain grid I9 below cut-off with respect to cathode is. No current, then, flows in the plate circuit of tube [6 and relay 26 becomes de-energized, allowing contacts 31 to open and interrupt the burner motor and valve circuits.

. If the cover 45 of the control housing is removed during the period when phototube 29 is illuminated and tube I6- is conducting in alternate half cycles, switch 36 is opened disconnecting screen grid 20 from tap 33. The screen grid, Which is connected to the cathode [8 through a resistance 35 assumes substantially the same potential as cathode [8, cutting 011 current through tube l6. Relay 26 becomes de-energized shutting down the burner. Since the power is not cut off, either to heater [1, or to the photocell circuit, the cathode of tube l6 remains in emissive condition and no biasing charge is built up on capacitance 28. When the cover is replaced, closing switch 36, tube 16 immediately becomes conductive, energizing relay 26. Resistance 35 is of a, value, for example, of the order of 100,000 ohms which is low compared to the resistance of a possible leakage path across the tube base or socket from the screen lead to ground, as the resistance of such a path, even under severe leakage conditions, is not likely to become less than one megohm. As a result, screen grid 35, even under such a condition of leakage, will assume a potential near enough to cathode potential to cut oil current through the tube I6 when switch 36 is open. The value of resistance 35 is high enough, however, so that the presence of resistance 35 does not add appreciably to the screen current flow through switch 36. Since the screen current is small, even when the tube is conductive, and is non-existent when the phototube is dark and grid I 9 below cut-off, it is apparent that switch 36 interrupts only a small current so that a simple and inexpensive type of switch will give satisfactory service. Since one side of the switch is grounded moreover, the cover itself, if of metal or a conductive projection directly attached thereto, may serve as one of the switch contacts.

Since certain changes may be made in the above-described article and different embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative only and not in a limiting sense.

What is claimed is:

1. Fire safetyapparatus partially enclosed within a housing having a removable cover, comprising a photocell for supervising a fire within a combustion chamber, said photocell being enclosed within said housing, means for controlling the flow of fuel into said combustion chamber, a tube including first and second control space paths and a main space path, said photocell being connected to said first control space path whereby the input potential to said space path is responsive to fire within said chamber, a power source for supplying the energizing currents for said tube and photocell, said fuel controlling means being connected to and responsive to the flow of current in said main space path, an impedance element directly shunting said second control space path, and a switch connected so as to interrupt only the power supply circuit for said second control space path, said switch being arranged for mechanical coupling to the cover of said housing whereby removal of said cover with respect to said housing opens said switch, thereby infiuencing the current flow in the main space path of said tube and preventing the how of fuel into said combustion chamber whenever said photocell is exposed to light sources from without said combustion chamber clue to removal of said cover.

2. Fire safety apparatus, partially enclosed within a housing having a removable cover, comprising a photocell for monitoring fire within a combustion chamber, said photocell being enclosed within said housing, a tube including anode, cathode, control grid, and screengrid elec trodes for generating an electrical output responsive to the presence or absence of fire within said combustion chamber as determined by said photocell, said photocell being connected to said control grid-cathode space path so as to vary the current flow therein in response to varying light conditions at said photocell, a power source for supplying the energizing currents for said tube and photocell, means responsive to said electrical output for controlling the fuel supply to said combustion chamber, an impedance element connecting said cathode directly to said screen grid, and a switch connected only between the power supply source and said screen grid, said switch being mechanically coupled to said cover whereby removal of said cover from said housing opens said switch, thereby substantially cutting ofi the current flow to the screen grid and consequently influencing the electrical output of said tube so as to terminate the flow of fuel into said combustion chamber whenever said photocell is exposed to light sources from without said combustion chamber due to removal of said cover.

E. CRAIG THOMSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

